Television receiver brightness and contrast control



Patented Apr. 25,* 1950 TELEVISION RECEIVER BRIGHTNESLS AND CGNTRAST CONTROL Hubert n. shaw, Drexel Hin., Pa., assign@ to Radio Corporation of America, a corporation of .Delaware Application May 21, 1948, Serial No. 28,376

3' Claims. (Cl. 178--7`.5)

The present invention relates to television, receivers and. more, particularly,. but not necessarily exclusively, to improvements in the video channel of a televisionv receiver whereby improved control. of the operation of the image producing device is provided.

The generalaim or object ot the present invention is4 to provide novel meansV for obtaining the desired contrast and-brightness. of a luminous image ina television receiver which is produced under' control of a video signal.` In accordancer with the invention,v they video amplier, or at least one stage of thevideo amplifier, amplies the D. C. (di-rect current) componentfof` the video signal. To this end, the amplier is physically or conductivel-y coupled to the image producing device, of the receiver. The video signal in apparatus embodying. the invention is preferably maintained at a. desired level in any suitable way, for exampleby an automatic gain control device. Contrast and brightness. are then controllable in accordance with the invention by a single. control which acts on the input to the image producing device.

AnotherV aim or obi'ect of. the. invention is to provide novel means for settingthe black level or input level for the. darkest part of a scene, object. or piece of. scanned subject matter.

A. further aim. or object of the invention, refe-rredV to above in broader terms, is. to provide a novel signal setting device which can be set to select a desired brightness of the produced image in. a television receiver while maintaining. its. apparent contrast..

A still further object is to provide, in a novel manner, for obtaining an image tube input sig.- nal having a substantiallyl fixed buty adjustable limit. and which varies. from the limit. through a desired range..`

Other objects4 and advantages of the present invention will. of course7 become. apparent and Vimmediately suggest themselves tothosey skilled in thle art to whichA the. invention is directed from a,l reading of the following specification connection with the accompanying, drawing which shows,V diagrammatically,. onel suitable form ci televisionA apparatus. embodying the invention..

Referring now tothe drawing, there is; illustrated a portion of a television receiver of a sui-table kind; for example, of the superhetercdyne type, comprising the usualrst detecten. a tunable oscillator,` and an intermediate frequency (L E.) amplifier.. none of 'which are shown as they are not a part of the present invention.. The

' Signal receiving equipment, :lustl mentioned or Y. its. equivalent',vr provides a signal for a video dcmodulator or second detector I6 shown, by way of example, as a diode.v The output of the second detector i6 is, physically connected to a video amplifier [8, shown by way of example' as comprising two physically connected tubes i9 and 2li. The output, of the tube 20; is applied by way of a physically connected coupling arrangement, having important features of this invention, to the control' electrode 26 of an image producing cathode ray' tube 28, shown schematically and by Way of example. This image producing tube may be a Kinescope having appropriate image. producing characteristics. The physically connected coupling arrangement between the tube 20 and the control electrode 2t of the image producing device includes a connection 29 and associated circuits which will be described more in detail, hereinafter. The cath- 0de 3| of the image producing tube 28 is connected to the. movable contact 32 ofv a potentiometer 3'3. The resistor 34. of this potentiometer is connected between a reference point of the circuit, such as the ground, and a suitable point of positive potential. on a voltage supply source (not shown).

-The output from the I. E. ampliner is delivered to the second detector i6 through a blocking condenser 36 and. a tunable impedance, for example, the inductance 3l whichv is tunable by a movable magnetic core 3.8 of powdered iron or the like,r It will be understood that a resonant circuit. may exist and serve as the coupling means between, the I. F. ampl'ier andthe second detector IB. This. resonant circuit, if it is present in the receiver which' embodies this invention, includes the tunable ind'uctance 31, the interelectrode capacity of the last stage tube (not shownl and the I. FK ampli-fier, and. other circuit capacities including the second detector I6.

Series andv shunt` peaking, coilsI 39 and 4l are arranged. in. the. detector circuit from between which the demodulatedV output is. applied. by way of a physical or. conductive connection 42 to the grid 43 ot the tube i9. The. output of the second detector being, taken from the anode of the tube I6 provides. that sync polarity is in the negative direction as shown conventionally by the wave form. M above. the coil 39. The output signal from the second, detector l 6' includes-blanking signaisy i6 and horizontal or line sync pulses 48 as well as the image signal portion. 49 of a composite signal. applied to the television receiver. It. will be understood that at appropriate intervals vertical'. or field sync pulses (not shown) will occur. v

The output of the tube IH is directly coupled to the grid 52 of the tube 20 through a physical or conductive coupling 53. Series and shunt peaking coils 56 and 5'I respectively may be included in a coupling network between tube I'9 and the grid 52 of the tube 20, if desirable or necessary, in order to avoid loss of high frequency components of the signal. The signal appearing at the grid 52 of the tube 29 is substantially a replica of the plate-current wave of the tube I9 but is a voltage wave 44a in the positive direction.

The plate circuit of the tube 29 includes a series peaking coil 62 and signal output from the tube 20 is of such polarity as to produce cutoff of the image tube 28 upon occurrence of blanking signals 48. A portion of the composite video signal output is taken off over a connection 'I6 and is applied to a sync separator (not shown) which provides a separated sync signal suitable for operating appropriate components of the apparatus (not shown) for producing a scanning action of the cathode ray beam in the image tube 28.

To secure operation of the illustrated television receiver in the most effective manner for purposes of the invention, an automatic gain control (A. G. C.) system is provided. The A. G. C. system is shown conventionally at Y18, and its input connection I9 is preferably in communication with a suitable point in the television apparatus so that the demodulated video signal is supplied to it. In the illustrative example, the connection 'i9 is taken from the output of the first stage tube I9 of the video amplier i8. The control signal derived, or generated, in operation of the A. G. C. system appears in a connection 8l, which is in suitable communication with the I. F. and R. F. amplifier, or either one. A suitable A. G. C. system is disclosed in Martinelli Patent No. 2,296,393, granted September 22, 1942. A preferred A. G. 1C. system is disclosed in a copending application of Edwin L. Clark, Serial No. 613,772, filed August 3l, 1945 and entitled Automatic gain control system.

The electrodes of the successive tube in the ampliiier stages are connected to a suitable voltage source as indicated schematically on the drawing. Inasmuch as the amplifier I8, having a direct current interstage coupling, is physically or conductively coupled to the detector i5 and the image producing tube 28, the electrodes of the tubes are connected to the voltage sources (not shown) having different values. The voltage values are assumed solely for illustrative purposes and convenience in describing the invention and its operation. For example, the cathodes of the detector I 6 and the amplifier tube I9 are indicated as being connected to a negative potential point 83 on the voltage supply source. Solely by way of example, this voltage is indicated as being -125 volts. The anode of the iirst stage amplier tube I9 is indicated as being connected to a point 86 on the voltage supply source, which is positive with respect to the connection of the cathodes of the rst two tubes just mentioned. Solely by way of example, this voltage is indicated as being -65 volts. The anode of the amplifier tube I9 is, therefore, positive with respect to its cathode. The anode of the second stage tube 20 of the video amplifier is indicated as being connected through a resistor 88 to a point 89 of positive potential on the voltage supply source. Solely by way of example, this point is indicated as being l-300 volts with respect to ground or some other voltage reference 4 point in the receiver. The method of connecting voltage supply sources and the like, and tube electrodes in a direct-coupled amplier is by now well known, and it is believed that the diagrammatic indication given in the drawing will be suicient to indicate the nature of the tube circuits employed to those skilled in the art. The connection 29 to the grid 26 of the image producing tube 28 is also connected to the movable contact 93 of a potentiometer 96, the resistance element 98 of which is connected between a voltage reference point in the receiver circuit, such as ground, and to the end of the resistor 88 which is in communication with the anode of the tube 20.

The operation of the system, including the invention, will now be described. The reference to voltage levels in the following is solely for convenience of description. Voltage values and the sign of each voltage will be determined by the apparatus selected for carrying out the invention and the voltage reference point for the circuits. in the illustrative example, ground is selected as the voltage reference point. The instantaneous voltage on the grid 52 of the video amplifier tube 29 is, for example, -70 v. at the black level, indicated by the dotted line IUI on the diagram above this amplifier stage. The grid to cathode voltage is then, for example, -10 v. The A. G. C. system maintains the black level by maintaining the instantaneous plate voltage of the nrst video ampliiier at a selected level on the tips of sync. The instantaneous plate voltage of the iirst video amplifier tube I9 may be about -65 v. as indicated on the tips of sync. The resistor 88 is so selected that the plate voltage of the second video amplifier tube 20 upon occurrence of the black level is at approximately the potential of the reference point in the system, such as ground. At the black signal level, therefore, there is practically no voltage drop across the resistor 98 of the potentiometer 96, and with the contact 32 of the potentiometer 33 properly adjusted, the image producing tube 28 is dark at all settings of the contact point 93.

On a dead-white picture element, that is to say when substantially no carrier is being received and supplied to the detector tube I6, the plate current of the tube 29 is low, and its plate voltage in the illustrative example is approximately v. with respect to the circuit voltage reference point, such as ground. The brightness of the image producing tube on a dead-White image element is, therefore, dependent upon the setting of the contact point 93. The operation of the television receiver is thus simplified since, once the potentiometer contact 32 has been adjusted properly, combined control of both brightness and contrast of the produced images is obtained by adjustment of the single control 93.

Solely by way of example and for the sake of completeness of description, suggested values for the more important resistors will be indicated. With the voltages'suggested by Way of example in the foregoing and employing a type 6K6 tube in the second video amplier stage, which is represented by the tube 20 in the illustrative example, the resistor 88 may be in the neighborhood of 10,000 ohms. The potentiometer resistor 98 may have a value of 2,200 ohms.

Having now described the invention, what is claimed and desired to be secured by Letters Patent is the following:

1. In a television receiver, an ampliiier having 75 input and loutput circuits, means for applying voltages to said circuits, an image producing tube, control means for said image producing tube, said amplifier output circuit including a load resistor, a ypotentiometer connected from said load resistor to a potential reference point for said amplier circuits, means to maintain recurring signal level at the input of said amplifier, and` means whereby said potentiometer connection to said load resistor has the same voltage as said voltage reference point for the amplier circuits upon occurrence of said recurring signal level.

2. In a television receiver, an amplifier having input and output circuits, means for applying voltages to lsaid circuits, an image producing tube, control means for said image producing tube, said amplifier output circuit including a load resistor, a potentiometer connected from said load resistor to a potential reference point for said amplifier circuits, means to maintain a recurring signal level at the input of said amplifier, and means whereby said potentiometer connection to said load resistor has the same voltage as said voltage reference point for the amplier cir cuits upon occurrence of said recurring signal n level.

,3. In a television receiver, a physically coupled ampliiier, an image producing tube having a cathode and a control electrode, control means for both the cathode and control electrode of said image producing tube, said amplier having output means for connection to the control means for the control electrode of said image producing tube, means for applying a voltage to said output means, said control means for the control electrode of said image producing tube Ibeing in direct communication with the voltage reference point for the connections to said amplifier, means to maintain a predetermined signal level at the input to said amplifier, means for applying a voltage at the input of said amplifier, and means whereby said control means for the control electrode of said image producing tube is effective to set a selected range of values p between a black value and a White value of the signal at the input to said amplier.

HUBERT R. SHAW.

REFERENCES CITED The following references are of record in the ille of this patent: 

